Cast iron composition for automobile engine exhaust system

ABSTRACT

The present invention relates to a cast iron composition for an automobile engine exhaust system that is characterized by adding iron as a main component, and 2.9-3.8 weight % C, 4.0-4.5 weight % Si, less than 0.3 weight % Mn, less than 0.05 weight % P, less than 0.1 weight % S, 0.8-1.2 weight % Mo, less than 0.025 weight % Mg are further added. When the present invention is adapted to an automobile exhaust manifold or a turbine housing of a turbo charger, it is possible to enhance high temperature strength and high temperature heat resistance.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of Korean Application No. 10-2003-0035977, filed Jun. 4, 2003, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a cast iron composition for an automobile engine exhaust system. In particular, the present invention relates to a cast iron composition for an automobile engine exhaust system capable of achieving excellent oxidation resistance and high temperature strength under high temperatures, in which the cast iron composition for an automobile engine exhaust system comprises Fe as a main component with C, Si, Mn, P, S, Mo and Mg as the other components. The cast iron composition of the present invention can be adapted to an exhaust manifold or turbine housing in an engine exhaust system.

BACKGROUND OF THE INVENTION

[0003] Generally, an automobile exhaust system is designed to reduce exhaust interference in order to prevent a decrease in engine performance. Namely, the exhaust system is made of a material having less heat capacity so that the exhaust system's temperature is quickly increased to a temperature in which the catalyst operates, in order to decrease the amount of harmful gas components being discharged, such as HC, CO, etc., after the engine starts. In addition, since high temperature exhaust gas passes through the exhaust system at 700-950° C., the material used in the exhaust system basically requires high temperature strength, high temperature fatigue strength, creep strength, high temperature oxidation resistance, etc. Materials most generally used for fabricating an exhaust manifold in automobiles include a high nickel nodular graphite cast iron in austenite, which has a high amount of nickel. For example, according to the Japanese Patent Laid-open No. Hei 6-228712, which is incorporated herein in its entirety, austenite cast iron includes 8-20 weight % Ni, which has excellent high temperature strength. However, the aforementioned material cannot be made thin, due to certain limitations in heat resistance. Therefore, it has a large heat capacity, so that the amount of exhaust gas is increased when the engine starts due to light-off-time of catalyst.

[0004] Therefore, a ferrite stainless cast iron material having a smaller heat expansion coefficient and excellent heat resistance is generally used for the above mentioned reasons. The above ferrite stainless steel has a lower cost as compared to austenite stainless steel, but has disadvantages in workability, flexibility, high temperature strength, etc. According to the Japanese Patent Laid-open No. Sho 61-261460, which is incorporated herein in its entirety, in order to overcome the above-described problems, boron (B) is added. Adding boron results in heat resistance and corrosion resistance problems. According to the Japanese Patent Laid-open Nos. Hei 2-175843 and Hei 3-274245, which is incorporated herein in its entirety, they suggest adding relatively more Mo and Nb during the fabrication of ferrite stainless steel to improve corrosion resistance and heat resistance. However, the above-described elements are expensive, and the use of stainless cast iron material presents various disadvantages as described above. Therefore, the actual use of the same in industry is limited.

[0005] In order to overcome the above problems, according to the Korean Patent Application No. 1997-57271, which is incorporated herein in its entirety, a SUS stainless cast iron material is disclosed, in which titanium and aluminum are added instead of tungsten and niobium and the amount of Si is increased. This cast iron material has a problem of differentiating itself from conventional cast iron.

SUMMARY OF THE INVENTION

[0006] Embodiments of the present invention provide a cast iron composition for an automobile engine exhaust system that enhances reliability and minimizes catalyst damage due to oxidation scale such that the addition of Si and Mo in an alloy used in an automobile engine exhaust system, namely, the components of nodular graphite cast iron used at high temperatures is properly adjusted, to thereby enhance high temperature oxidation resistance, high temperature strength, and casting properties.

[0007] To achieve these objects, the present invention provides a cast iron composition for an automobile engine exhaust system that is characterized by adding iron as a main component, and 2.9-3.8 weight % C, 4.0-4.5 weight % Si, less than 0.3 weight % Mn, less than 0.05 weight % P, less than 0.1 weight % S, 0.8-1.2 weight % Mo, less than 0.025 weight % Mg are further added.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The aforementioned aspects and features of the present invention will be explained in the following description and in connection with the accompanying drawings, wherein:

[0009]FIG. 1 is a histogram of a result of a high temperature tensile strength test of one embodiment and a comparison example of the present invention;

[0010]FIG. 2 is a graph of a result of a high temperature oxidation resistance test of one embodiment and a comparison example of the present invention;

[0011]FIG. 3 is a photomicrograph of a thick part of cast iron fabricated according to one embodiment of the present invention;

[0012]FIG. 4 is a photomicrograph of a port part of cast iron fabricated according to another embodiment of the present invention;

[0013]FIG. 5 is a photomicrograph of a thick part of cast iron fabricated based on an example in the conventional art; and

[0014]FIG. 6 is a photomicrograph of a port part of cast iron fabricated based on an example in the conventional art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0015] Generally, cast iron used in automobile engine exhaust systems is formed from an alloy made of C, Si, Mn, P, Mg and Mo. In the present invention, the addition of Si in the above composition enhances oxidation resistance of the material in comparison to conventional cast iron for an automobile engine exhaust system. The addition of Mo in the present invention enhances high temperature strength by enhancing the matrix based on ferrite ratio reduction and carbide formation.

[0016] Here, Si is preferably added in a range of 4.0-4.5 weight %. If the addition of Si is out of the above range, namely, if the addition is below 4.0 weight %, oxidation resistance is decreased such that there is no difference in oxidation resistance between the conventional art and the present invention. If the addition of Si exceeds 4.5 weight %, the flexibility of cast iron ware is sharply decreased, resulting in many problems in the main composition. Therefore, it is preferable to have the addition of Si be limited to a range of 4.0-4.5 weight %.

[0017] Furthermore, Mo is preferably added in a range of 0.8-1.2 weight %. If the addition of Mo is below 0.8 weight %, high temperature strength is poor. In the case that the addition of Mo exceeds 1.2 weight %, the amount of pearlite is increased, so that hardness and tensile strength are increased. On the contrary, elongation and impact degree are decreased.

[0018] In addition to the above-described components, the present invention also comprises 2.9-3.8 weight % C, less than 0.3 weight % Mn, less than 0.05 weight % P, less than 0.1 weight % S, 0.025-0.045 weight % Mg. If the addition of C is less than 2.9 weight %, a shrinkage defect occurs. If the addition of C exceeds 3.8 weight %, a carbon flotation phenomenon occurs. The addition of Mn is less than 0.3 weight % because it is added as one of five basic components of cast iron. Mn operates to promote pearlite. If Mn is over added, the material may become brittle. The addition of P is less than 0.05 weight % because it is also added as one of five basic components of cast iron. If the addition of P exceeds 0.05 weight %, carbide may appear. Therefore, it is preferable to add P less than the above range. The addition of S is less than 0.1 weight % because it is added as one of five basic components of cast iron. If the addition of S exceeds 0.1 weight %, inclusions may occur. Therefore, it is preferable to add S in quantities less than the above range. Finally, Mg is added in a range of 0.025-0.045 weight %. If the addition of Mg exceeds this range, the material's strength is increased, but its flexibility is decreased. In contrast, if the addition of Mg is less than this range, the material's flexibility is increased, but its strength is decreased.

[0019] Table 1 shows the components of one embodiment of the present invention and comparison examples 1 and 2. TABLE 1 Chemical components (wt %) Section Section C Si Mn P S Mo Cu Mg Y-block Embodiment 3.00 4.24 0.22 0.031 0.006 0.85 0.07 0.032 (HighSiMo) Comparison 3.51 3.77 0.25 0.031 0.002 0.42 0.06 0.034 example 1 (FCD50HS) Comparison 3.34 3.62 0.27 0.034 0.002 0.03 0.08 0.03 Example 2 (FCD-H)

[0020]FIGS. 1 and 2 respectfully show the views results of a high temperature tensile strength test and a high temperature oxidation resistance test of one embodiment of the present invention and the comparison examples. As seen in Table 1, the tensile strength and high temperature oxidation resistance are excellent in the present invention when compared to the comparison examples.

[0021] During the high temperature tensile strength test, when the test temperature was fixed at 700° C. through 800° C., as the temperature increased, the tensile strength decreased. In this embodiment of the present invention, the tensile strength was 120-60 Mpa, and the tensile strength in comparison example 1 was 100-50 Mpa. In addition, the tensile strength in comparison example 2 was 70-40 Mpa. During the high temperature oxidation resistance test, the decrease degree of the oxidation was about 110 g/m², 235 g/m², and 290 g/m² in this embodiment of the present invention, in comparison example 1, and in comparison example 2, respectively.

[0022] The following Table 2 shows the ferrite ratio and hardness of another embodiment of the present invention and the comparison examples. In the cast iron of the present invention, the ferrite ratio of the thick part (relatively thick portion in a corresponding product) and the port part is controlled and set higher than at least 80%. By analyzing the sample, the above condition was known to be satisfied irrespective of the portions. In these embodiments of the present invention, it is possible to enhance the material's hardness by about 30% as compared to the comparison examples based on ferrite ratio reduction and carbide generation when adding too much Mo.

[0023] Therefore, in the present invention, the cast iron composition is properly adjusted. In particular, the additions of Si and Mo are different, which enhances high temperature strength and high temperature heat resistance as compared to the conventional high nickel nodular graphite cast iron material and SUS stainless cast iron material for the fabrication a new cast iron material for an automobile engine exhaust engine system. TABLE 2 Section Ferrite (%) Spherocity (%) Hardness (HB) Embodiment Thick part 82.8 83.6 223-229 (HiSiMo) Port part 87.5 84 230-233 Comparison Thick part 87.6 85.9 185-187 example 1 Port part 90.6 97.3 195-200 (FCD 50HS)

[0024] As described above, in a cast iron composition for an automobile engine exhaust system in according to the present invention, it is possible to enhance physical properties such as a high temperature oxidation resistance, a high temperature strength, etc. by about 50%, 20% as compared to a conventional nodular graphite cast iron material. The cost is low, as compared to a conventional SUS stainless cast iron, for the fabrication of an economical and practical alloy for an engine exhaust system in the present invention.

[0025] While the foregoing description represent various embodiments of the present invention, it should be appreciated that the foregoing description should not be deemed limiting since additions, variations, modification and substitutions may be made without departing from the spirit and scope of the present invention. It will be clear to one of skill in the art that the present invention may be embodied in other forms, arrangements, proportions, and using other elements, materials, and components. For example, although a cast iron composition for an automobile engine exhaust system is described, the cast iron composition may be adapted to the physical conditions of an application different than an automobile engine exhaust system by varying the proportion of each component. The present disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims and not limited to the foregoing description. 

What is claimed is:
 1. A cast iron composition for an automobile engine exhaust system that is characterized by adding iron as a main component, and 2.9-3.8 weight % C, 4.0-4.5 weight % Si, less than 0.3 weight % Mn, less than 0.05 weight % P, less than 0.1 weight % S, 0.8-1.2 weight % Mo, less than 0.025 weight % Mg are further added. 